Sorbent re-circulation system for mercury control

ABSTRACT

A flue gas purification system comprising in the flue gas stream an electrostatic precipitator, mercury sorbent injection and a two-stage electrostatic separator, and wherein the mercury sorbent injection is positioned between the electrostatic precipitator and the two-stage electrostatic separator.

RELATED APPLICATION

This application is a division of Ser. No. 10/272,973 filed Oct. 16,2002; and is now U.S. Pat. No. 7,060,229.

FIELD OF THE INVENTION

The herein disclosed invention finds applicability in the field of fluegas purification, resulting particularly from the burning of coal.

BACKGROUND OF THE INVENTION

Sorbent addition to the flue gas in the duct ahead of an ESP(Electrostatic Precipitator) is a process that is being used as a methodto reduce mercury emissions from coal burning power plants. As currentlypracticed a sorbent, such as activated carbon, is added to the flue gasupstream of a particulate control device, either an electrostaticprecipitator or fabric filter. If the sorbent is added ahead of aprecipitator, mercury removal is limited to 60-70%, and higher rates ofsorbent addition are needed to produce these levels of mercury removalthan with baghouses. The inefficiency of this process results from thetypical short flue gas residence time between the point of addition ofsorbent and the ESP. This prior art process also mixes the sorbent withthe collected fly ash, and the carbon in the ash can limit its use as aningredient in concrete. If the sorbent is added to the flue gas after itleaves the ESP and before a “polishing” baghouse, better sorbentutilization is achieved and high mercury removal rates result, but thepolishing baghouse can be an expensive item. An alternate approach thatovercomes these disadvantages is described in this invention. The hereindisclosed invention utilizes a two-stage electrostatic precipitator orelectrostatically enhanced separator (EES), which is a particulatepolishing device developed by EPRI. This two-stage electrostaticprecipitator is a mechanically augmented electrostatic device thatseparates flue gas into two separate gas streams; a clean stream thatcontains very little ash and a bleed stream that contains most of theash.

PRIOR ART PATENTS OF INTEREST

Ochi (U.S. Pat. No. 6,149,713) teaches a flue gas treating processemploying an electrostatic precipitator introduced into the absorptiontower of a desulfurizer based on lime-gypsum. Flue gas discharged fromthe boiler is introduced to an air heater and air supplied to the boileris heated by the flue gas and the flue gas is then sent to a heatrecovery unit. The cooled flue gas is sent to an ESP. Flue gas leavingthe ESP is desulfurized and the flue gas is then discharged.

Chang (U.S. Pat. No. 5,505,766) teaches a method for removing pollutantsfrom flue gas by using a sorbent in a baghouse having a filter bagtherein. The method involves having the flue gas exit through an outletduct. An electrostatic precipitator is used in a method along with abaghouse. A sorbent is used for the baghouse filters. The filters areperiodically recoated with sorbent. Note, particularly, sorbent suitablefor mercury removal is employed (col. 4, line 50).

Chang (U.S. Pat. No. 5,158,580) teaches a particle collector forremoving pollutants from flue gas. An electrostatic precipitator is usedin the process as well as a filter. A baghouse can be employed in theprocess of flue gas purification.

Wojtowicz (U.S. Pat. No. 6,103,205) teaches the use of activated carbonto absorb mercury from flue gas. A process for recovering the mercuryfrom the absorbent is also disclosed.

None of the prior art patents teach the system for removing mercury astaught by the herein disclosed invention.

BRIEF SUMMARY OF THE INVENTION

The new technology takes advantage of the two-stage electrostaticprecipitator's ability to concentrate particulate matter into a fractionof the flow that the device is treating. In fact, if the particulatematter has a diameter of 10 μm or greater, the separation efficiency isextremely high, well over 90%. Sorbents used for mercury capture can beproduced with effective diameters that are 10 μm or greater and hence,these sorbents would be concentrated with high efficiency into the bleedstream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a typical prior art flue gaspurifier system.

FIG. 2 is a schematic representation of a flue gas purifier employingthe sorbent re-circulation system for mercury control of this inventionemploying re-circulation and particulate fabric filter.

FIG. 3 is a schematic representation of an alternative embodiment ofthis invention involving mercury sorbent injection along with atwo-stage electrostatic separator and particulate fabric filter.

FIG. 4 is a schematic representation of a further alternative embodimentof the invention involving mercury sorbent injection with re-circulationfrom the two-stage electrostatic separator to the ESP and also back tothe two-stage electrostatic separator.

FIG. 5 is a still further schematic representation of an alternativeembodiment involving mercury sorbent injection with an alternative flowfrom the two-stage electrostatic separator.

FIG. 6 is a figure illustrating a two-stage electrostatic precipitatorthat can be used in carrying out the herein disclosed invention.

Exemplifying two-stage electrostatic precipitators are Altman et al U.S.Pat. No. 6,096,118 entitled “Electrostatic Separator for SeparatingSolid Particles from a Gas Stream” herein described below, and Altman etal U.S. Pat. No. 5,683,494 entitled “Electrostatically EnhancedSeparator (EES)”.

DESCRIPTION

The concept of this invention is illustrated in FIGS. 1-6. FIG. 1illustrates how the prior art two-stage electrostatic precipitator isused as particulate polishing device. For this purpose, a two-stageelectrostatic precipitator is inserted into the duct work between anexisting ESP and the stack. Typically, 10% of the total flow enteringthe two-stage electrostatic precipitator would be returned to the inletof the ESP for final collection of the particulate matter concentratedin this stream. The clean flow is sent to the stack. In this process,the particulate matter is “trapped” in a loop until it is finallycollected in the ESP.

With reference to FIG. 2, there is shown a schematic representation ofthis invention which has a sorbent re-circulation system for mercurycontrol. The system encompasses an electrostatic precipitator (ESP) upstream from a two-stage electrostatic separator. A mercury sorbentinjection device is placed between the ESP and the two-stageelectrostatic separator. This two-stage separator is anelectrostatically enhanced separator (EES) which provides mechanicalseparation as well as electrostatic separation and provides a compactand highly efficient separation system. The two-stage electrostaticseparator (or EES) is described in U.S. Pat. No. 6,096,118 and in U.S.Pat. No. 5,683,494 (see FIG. 6). The entire contents of these patentsare herein incorporated by reference.

In greater detail FIG. 2 describes a schematic representation of thesorbent re-circulation system for mercury control which has flue gasfrom an air heater entering an electrostatic precipitator. Once the fluegas leaves the electrostatic precipitator, it passes through a mercuryabsorbent where mercury present in the flue gas is absorbed. Asunderstood by this invention, the mercury absorbent (e.g., activatedcarbon or powdered clays) is blown as a powder through jets to absorbthe mercury. Upon passing the mercury absorbent area, the flue gaspasses through a two-stage electrostatic precipitator wherein a part ofthe flue gas which has been adequately cleaned passes up the stack and asecond part of the flue gas, namely a bleed flow of the flue gascontaining particles leaves the two-stage electrostatic precipitator.Part of this bleed flow (20% to 50%) is sent back to reenter thetwo-stage electrostatic precipitator and part is sent through a fabricfilter particle collector and then to the stack. Sorbent injectionprobes (e.g., powder injection) are added to the duct work connectingthe ESP to the two-stage electrostatic precipitator. The bleed flow isincreased from the nominal 10% level to 20 to 50% of the inlet flow. Afraction of the bleed flow, equivalent to 10% of the flow, that wouldhave entered the two-stage electrostatic precipitator withoutre-circulation is sent to the small particulate collector, preferably afabric filter. The rest of the bleed flow is re-circulated to the ductleading to the two-stage electrostatic precipitator. In this case, thesorbent is trapped in this re-circulation loop and its concentration inthe flue gas is greatly increased. This increase in concentrationenhances mercury removal and improves sorbent utilization. Since thesorbent is collected in a device different from the ESP, it does notcontaminate the fly ash collected in the ESP. Thus, the fly ash can besold as a commercial byproduct.

With reference to FIG. 3, an alternative embodiment of the mercuryadsorption system is described. In this system, flue gas from the airheater enters the electrostatic precipitator (ESP or primary particulatecollector) and exits partially cleansed. Upon leaving the ESP, thecleansed flue gas is treated by the injection of a mercury sorbent whichabsorbs the mercury in the stream. Most of the treated flue gas leavesthe two-stage electrostatic separator as a clean flow and goes to thestack. A second portion leaves the two-stage electrostatic separator asa bleed flow (5 to 30%) and is sent a particulate collector (e.g.,fabric filter) and then to the stack.

With regard to FIG. 4, a further alternative embodiment of the mercurysorbent system of this invention has sorbent injection between theelectrostatic precipitator (ESP) and the two-stage electrostaticseparator (EES, electrostatically enhanced separator). FIG. 4 describesa process by which flue gas from an air heater passes through an ESP istreated with a mercury adsorbent such as charcoal and sent to atwo-stage electrostatic separator (EES). A major portion of the flue gasleaving the EES is clean and goes as the clean flow to the stack. Ableed flow portion (20 to 50%) is divided into a first portion,re-circulation flow (equivalent to 10 to 40% of inlet flow) is returnedto the two-stage electrostatic separator for further cleaning and asecond, final bleed flow (equivalent to 10% of inlet flow) is returnedto the ESP.

Another alternative schematic representation of the mercury sorbentre-circulation system of this invention (FIG. 5) has the flue gas fromthe air heater entering the ESP to be cleaned. In the ESP particles fromthe flue gas are trapped by the plates of the ESP and then depositedinto hoppers (not shown). The flue gas leaves the ESP and is treated bymercury sorbent injection and enters the two-stage electrostaticseparator (EES) wherein a clean portion (or clean flow) goes to thestack. A second portion leaves the EES as a bleed flow (5% to 40%) andis returned to the ESP for retreatment.

With regard to FIG. 6, the preferred two-stage electrostatic separatoror electrostatically enhanced separator (EES) is exemplified in U.S.Pat. No. 5,683,494 to Altman et al, the entire contents of which isincorporated herein. The device includes a cylindrical separation vessel11 having a thin elongated inlet passage 13 for admitting the solidparticle and/or mist laden gas stream 14. The separation vessel 11 islikewise provided with a thin elongate outlet passage 15 for expellingthe separated particles in the form of a concentrated “bleed flow” 16.Both the inlet passage 13 and the outlet passage 15 are in fluidcommunication with the separation vessel 11 and maintain a tangentialfluid flow with respect to the walls 17 of the separator chamber 11. Itis important that both the inlet passage 13 and outlet passage 15 beformed as narrow slits to distribute the fluid flow lengthwise betweenits opposing ends along the cylindrical walls of the separation vessel11. This insures that all particulates enter and exist the separatorvessel 11 proximate to the walls 17 thereof, and such proximity, first,greatly improves the separator (EES) 10 separation efficiency becausethe turbulent diffusion processes causing the particulate re-entrainmentare less intensive in the region adjacent to the separator walls 17, andsecondly, eliminates the corona suppression problem due to very lowparticulate concentration in the EES core.

The separator 10 further includes two vortex finders 18 which are formedas cylindrical tubes.

A discharge electrode 19 extends centrally throughout the vortex finders18. A power supply is connected between the walls 17 of the separationvessel 11 and the discharge electrode 19 for establishing an electricpotential therebetween which serves to charge the particles entrained inthe separation vessel 11 and to repel charged particles toward theseparation walls 17.

Gas stream 14 enters the elongate inlet passage 13 and is introducedtangentially into the separator vessel 11. This creates a vortex insidethe separation vessel 11. As the particles are swirled in the separationvessel 11, the inertia of the heavier particles will propel them outwardtoward the wall 17 of the separation vessel 11. The particles willremain in the separator 10 for at least 180° revolution, and willeventually be expelled from the separator 10 through the outlet passage15. This mechanical separation is electrostatically enhanced by thedischarge electrode 19. A voltage potential is applied to the dischargeelectrode 19 with respect to the outer walls 20 of the separation vessel11, and this forms an electrostatic field within said vessel 11, wherethe particulate charging and electrostatic separation are provided. Thepolarity of the potential applied to the discharge electrode 19 is thesame as the charge imparted on the particles. This way, theelectrostatic field repels the particles and their entry into theseparation core is prevented. At the same time, sanitized gas 21 is freeto flow outward through the clean gas outlets 22.

The inertial separation coupled with electrostatic separation in theseparation vessel 11 results in a very pure clean gas stream 21 flowingfrom the clean gas outlets 22. In contrast to the existingelectrostatically enhanced cyclones, all particles enter and exit theseparator vessel 11 proximate the separator walls 17, and particulateseparation processes are less intensive than that in the separator core.That predetermines very high EES separation efficient.

As an alternative embodiment of this invention, the inventorcontemplates employing a two-stage electrostatic separator as defined inU.S. Pat. No. 6,096,118 to Altman et al, the entire contents of which isherein incorporated by reference. The patent sets forth a two-stageelectrostatic separator for separating particles from a particle ladengas stream and includes a pre-charging section and spaced-apartgas-permeable grounded, discharge electrodes charged at oppositepolarities and defining a separating section between them. The groundedelectrode being positioned upstream from the discharge electrode. Theparticles in the particle laden gas stream are pre-charged to a certaincharge in the pre-charging section and penetrate through the groundedelectrode into the separating section, where the particles are separatedfrom the particle laden gas stream. As a result, a clean gas streamexits from the separating section through the discharge electrode, andthe particles separated from the particle laden gas stream are partiallycollected on the grounded electrode, and partially are removed with ableed flow which is re-circulated. The electrodes may be of linear orcylindrical shape, and linear electrodes may be arranged in a “zig-zag”order.

In the mercury sorbent system of this invention, the amount of mercuryabsorbent added is initially 1 to 10 pounds per million actual cubicfeet and as the process proceeds can build up to a factor of one to ten.

The mercury absorbed on the sorbent is either removed by the particulatecollector or by the electrostatic precipitator during re-circulation.

There are many advantages attendant to employing the system of thisinvention. For example:

1. The process of this invention overcomes the low efficiency of thesorbent addition before an ESP approach for mercury control.

2. The invention avoids the pressure drop problems associated withsorbent addition before a fabric filter.

3. With the use of the inventive process, there is retained the abilityto sell commercially acceptable fly ash.

4. The invention has the capability of providing very high mercuryremoval levels with good sorbent utilization low pressure drop (thepressure drop across the two-stage electrostatic precipitator isapproximately 0.5 inches of water).

5. The two-stage electrostatic precipitator with re-circulation can beadded after a primary particulate control collection device treating agas contaminated with mercury to provide effective mercury control.

Demonstration

The concept of this invention has been demonstrated in 1.5 MW pilottests at Alabama Power Company's Plant Gaston. Preliminary mercuryremoval tests indicated relatively high mercury removal at moderatesorbent addition rates.

Obviously, many modifications may be made without departing from thebasic spirit of the present invention. Accordingly, it will beappreciated by those skilled in the art that within the scope of theappended claims, the invention may be practiced other than has beenspecifically described herein.

1. A flue gas purification system comprising in the flue gas stream: 1)an electrostatic precipitator, 2) a solid mercury sorbent injectiondevice and 3) a two-stage electrostatic separator having a clean flowstream and a bleed stream and wherein the solid mercury sorbentinjection device is placed between the electrostatic precipitator andthe two-stage electrostatic separator such that in operation the solidmercury sorbent is directed to a treated stream between theelectrostatic precipitator and the two stage electrostatic separator;wherein a first portion of flue gas leaves the two-stage electrostaticseparator, goes to the clean flow stream and a second bleed flow portionis divided into a first divided portion and a second divided portion;wherein the first divided portion is returned to the two-stageelectrostatic separator and the second divided portion is returned tothe electrostatic precipitator.